The present invention relates to optical waveguide fibers that are suitable for use in optical telecommunication systems, more particularly to optical fibers that are particularly well suited for use in single mode operation.
Dispersion management is very important for Non-Zero Dispersion Shifted Fibers (NZDSF). In particular, it is desired to have small residual dispersion for the transmission system across the entire transmission band. Since the dispersion of NZDSF varies as a function of wavelength, dispersion compensation is desired. In order to compensate for slope of the NZDSF fiber with a fairly short fiber length of Dispersion Compensating (DC) fiber, a DC fiber having a large negative slope is desired. However, in designing a high slope DC fiber, several problems are encountered. These problems are: 1) Multi-Path Interference (MPI), Insertion Loss (IL), and dispersion linearity as a function of wavelength. A single-moded fiber solution is best for controlling MPI. However, the IL for typical single-mode fiber designs makes them generally unattractive. Dispersion linearity is also difficult to achieve in single-mode fiber designs. Thus, few-moded designs have been the desired solution that meet the IL and dispersion linearity requirements. However, these fibers generally have unacceptable MPI.
Consequently, it would be desirable to have an optical fiber that propagates in the fundamental mode LP01 without appreciable attenuation, while at the same time, filtering out the higher order modes such as LP02 and LP11. Such a fiber would exhibit single-moded operation.
In accordance with one embodiment of the present invention, an optical fiber is provided having a core, a cladding, and a coupling coating wherein the cladding-coating interface is located at a radius (Ri) less than 55 microns; more preferably less than 50 microns; and most preferably less than 45 microns from the fiber""s centerline. This embodiment advantageously enables Higher Order Modes (HOMs) (e.g., LP11, LP02 light propagation modes) to be efficiently filtered out upon sufficient bending of the optical fiber. For example, in operation, the HOMs will be filtered out when a sufficient length of the fiber is wound onto a sufficiently small diameter spool in a Dispersion Compensating (DC) module. In particular, the cladding thickness is reduced to less than 55 microns from a conventional thickness of 62.5 microns. Advantageously, the outer diameter of the fiber""s coating may be made smaller thereby resulting in a fiber with a smaller overall outside diameter thereby using lesser amounts of cladding glass and coating materials to manufacture. Additionally, longer lengths of optical fiber may be wound onto standard sized spools or the same lengths of such fibers may be packaged in a smaller volume. This is particularly useful for reducing the overall size of DC modules. By way of example, the amount of glass utilized in the fiber may be reduced by 20% to 75% and the amount of coating utilized may be reduced by as much as 30%. Thus, the present invention may be employed, for example, to manufacture DC modules at lesser cost and in significantly smaller packages.
One preferred optical fiber according to the present invention has a core, a cladding layer, a coupling coating abutting the cladding layer at a cladding-coating interface, the coupling coating has a refractive index higher than the cladding layer and the cladding-coating interface is positioned at radius of less than 55 microns from a centerline of the optical fiber such that higher order mode attenuation is enhanced as compared to fundamental mode attenuation, and a measured cutoff wavelength (xcexc) of the optical fiber is greater than 1500 nm.
According to further embodiments of the invention, the cladding-coating interface is positioned at less than 50 microns from the centerline of the optical fiber; and more preferably less than 45 microns from the centerline of the optical fiber. In accordance with preferred ranges, the cladding-coating interface is positioned at greater than 30 microns and less than 50 microns from the centerline of the optical fiber; more preferably greater than 35 and less than 50 microns from the centerline; and most preferably greater than 40 and less than 50 microns from the centerline.
In one preferred embodiment, the fiber is a Dispersion Compensating (DC) fiber wherein the core has a refractive index profile having a central core segment having a positive delta (xcex941), and a moat segment surrounding the central core segment having a negative delta (xcex942). The DC fiber preferably also includes a ring segment surrounding the moat segment having a positive delta (xcex943).
According to another embodiment of the invention, a dispersion compensating module is provided, comprising a winding spool, a dispersion compensating fiber wound onto the spool, the dispersion compensating fiber including a core, a cladding layer, a coupling coating having a refractive index higher than the cladding layer, and a cladding-coating interface at a point of interface between the cladding layer and the coupling coating, the cladding-coating interface being positioned at a radius of between 35 and 50 microns from a centerline of the dispersion compensating fiber, and the dispersion compensating fiber exhibits a measured cutoff wavelength (xcexc) greater than 1500 nm.
An optical transmission system is provided in accordance with another embodiment of the invention. The system comprises a transmitter, a length of optical transmission fiber optically coupled to the receiver, said length being greater than 10 km, a dispersion compensating fiber optically coupled to the transmission fiber, said dispersion compensating fiber including a core, a cladding layer, a coupling coating abutting the cladding layer at a cladding-coating interface, the coupling coating having a refractive index higher than the cladding layer wherein the cladding-coating interface is positioned at radius of less than 55 microns from a centerline of the optical fiber such that higher order mode attenuation is enhanced as compared to fundamental mode attenuation, and a receiver optically coupled to the dispersion compensating fiber.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.